This invention relates generally to digital radio communication systems using time division multiplexing (TDM) to convey paging and traffic information, and more specifically to a method and system for locating an optimum paging channel and providing synchronization in a Time Division Multiple Access (TDMA) communication system.
In an exemplary digital radio communication system, information is exchanged by a base station and one or more mobile stations using a TDM of traffic channels, paging control channels, and/or broadcast control channels in a technique called TDMA, in which successive time periods called frames are each divided into successive time slots. Each time slot is allocated to, for example, traffic information, paging information and/or broadcast information such that each frame comprises traffic time slots, paging time slots and/or broadcast time slots. In some TDMA systems, paging and broadcast information can be sub-multiplexed in one time slot, and the other time slots can carry traffic information. Each paging time slot includes paging information identifying which mobile station is being paged. Further details of such communication systems are available in the literature and are specified in the standards TIA/EIA/IS-95-A and TIA/EIA/IS-136, among others, that are promulgated by the Telecommunications Industry Association (TIA) and the Electronic Industries Association (EIA). Such systems are also described in U.S. Pat. No. 5,539,748 to Raith; and in U.S. patent application Ser. No. 08/768,976 filed on the same date as this application and entitled "Reduced Power Sleep Modes" by Dent et al.; among others.
FIG. 1 shows two successive TDMA frames XX, YY, each of which comprises 128 time slots sequentially numbered 1-128, that might be used in the exemplary digital radio communication system described above. Each time slot contains data symbols D, which for the purposes of this application may be considered arbitrary, and predetermined synchronization symbols S, as illustrated by the partial exploded view of time slot 6 of frame XX. In an exemplary communication system, the digital symbols D and S are modulated on a radio-frequency carrier signal at a bit-rate of approximately 1.625 megabits per second (Mb/s), preferably using .pi./4 quadrature phase shift keying (.pi./4-QPSK), and each frame is twenty milliseconds (20 ms) long. Thus, each frame includes 16250 quaternary symbol periods (data or sync), and each quaternary symbol represents two bits of information according to which of four signal vectors is transmitted (1, j, -1, or -j for even-numbered symbols and .+-.1.+-.j for odd-numbered symbols). The total number of information bits carried by each such TDMA frame is therefore 32500.
In the exemplary communication system, the 16250 symbol periods in a frame are divided into 128 time slots of 124 symbol periods each. Except for the last and first slots of successive frames, the members of each pair of successive time slots are separated by two symbol periods that are used for changing transmitter power levels between time slots; the last and first time slots are separated by 124 symbol periods. It will be appreciated that the extra long separations (124 symbol periods) can occur between any two time slots and that a frame's total of 378 symbol periods used for separating time slots can be distributed in other ways, for example to effect different relative slot timings. Nevertheless, corresponding time slots in successive frames should always occur at regular intervals of 20 ms, or 16250 symbols. TDMA communication systems currently in use generally have regularly spaced timeslots, but the above-described exemplary system is not so restricted and is therefore more general.
A mobile station operating in a TDMA communication system must synchronize itself in time and frequency with a paging slot or channel in order determine whether it is being paged. This has usually been done by having the mobile station first identify a paging time slot using a coarsely accurate timing and then correlate symbols received in the paging time slot with known synchronization symbols transmitted in the paging time slot to obtain fine timing accuracy. Paging time slots are often identifiable because they are transmitted (and, hopefully, received) at a higher average power level than other time slots. The mobile station identifies a paging time slot in a frame by identifying the time slot having the highest energy level and then achieves time-synchronization by correlating the known sync symbols with the sync symbols received in the paging time slot.
In the European digital cellular system known as GSM, base stations transmit an unmodulated TDMA burst to give receivers coarse sync and another TDMA burst that contains a long pattern of known bits to allow receivers to obtain fine sync. Having obtained coarse and then fine sync, a GSM receiver transitions to listening to yet another type of TDMA slot containing paging information. The GSM system is representative of the functions that must be performed to enable a mobile phone to obtain synchronization on power up, but GSM has limitations when applied to a satellite communications system. In particular, a satellite system may not be able to transmit an unmodulated burst to provide coarse sync.
Before time-synchronization can be achieved, a receiver must also "synchronize" itself to the frequency of the signal it is trying to receive. A TDMA frame to be transmitted by an earth-orbiting satellite may be modulated on a carrier signal having a frequency typically in the range 1.6 gigahertz (GHz) to 2.5 GHz, and the modulated carrier may be received by a terrestrial fixed, mobile, or hand-held receiver having a limited frequency accuracy. It will be appreciated that the frequency accuracy of a small, battery-powered, hand-held radio receiver is typically about .+-.5 parts per million (ppm) when the receiver is free-running, i.e., before the receiver detects the carrier signal, determines its relative frequency error, and corrects it. (This process is known as initial acquisition.) Thus, the frequency error during acquisition is .+-.5 ppm.times.2.5 GHz, or .+-.12.5 KHz (kilohertz). It is advantageous therefore to choose wideband TDMA waveforms, such as postulated for the exemplary system, in order that this frequency error be small in relation to the receiver bandwidth, thus facilitating initial acquisition.
The receiver amplifies and filters the collected signal energy, digitizes selected portions, and carries out further processing of the digitized portions, for the purpose of time synchronization for example. As described in U.S. Pat. No. 5,048,059 to Dent, digitization can advantageously be carried out using a logpolar technique, in which signals related to instantaneous amplitude and to instantaneous phase are digitized. The signals related to instantaneous phase, for example, can represent the cosine and sine of the phase angle, respectively, and the signals related to instantaneous amplitude can be proportional to the logarithm of the instantaneous radio signal strength indication (RSSI) signal that is well known in mobile communication systems. U.S. Pat. No. 5,048,059 is expressly incorporated in this application by reference.
The exemplary radio communication system disclosed in U.S. patent application Ser. No. 07/967,027, filed on Oct. 27, 1992, entitled "Multi-Mode Signal Processing" by Dent et al. includes a dual mode FDMA/TDMA mobile phone that obtains time and frequency synchronization, and that patent application is expressly incorporated here by reference. To achieve frequency synchronization, a method of correlating sync symbols having undetermined frequency errors with phase-twisted sync symbols, where the phase twists correspond to different, respective frequency errors, is described in Swedish Patent No. 459 137 to Raith, which is also expressly incorporated here by reference.
Achieving time synchronization (and frequency synchronization, for that matter) is more difficult when the ratio of the signal power to the noise power (SNR) is low. One problem with satellite communication systems is that the large distances between transmitters and receivers result in low SNRs, oftentimes even less than unity. Due to the low SNR of the received signal, it is difficult to identify accurately, in a single-shot manner, the time slot having the highest energy level in a frame; a traffic time slot may be improperly identified, rather than a paging time slot. Accordingly, simple-mindedly identifying the highest-energy time slot as a paging time slot can result in errors in satellite and other communication systems operating at low SNR.
A more reliable method would comprise averaging the slot energy over several frames. Unfortunately, at very low SNR the necessary averaging time may be long, such that significant timing drift would occur during the averaging period. Thus, an average of energy in slot 5 at the start of the averaging period may drift to become corrupted by energy from slots 4 or 6 at the end of the averaging period. "Dynamic programming" is a technique that has been applied successfully in automatic voice recognition devices, which involve sampling speech signals. In such devices, dynamic programming is used to choose a time warping that maximizes sums of correlations (or minimizes mismatches) between a sequence of spoken words and digitally stored examples, or templates. Since words can be spoken at various rates that typically differ from the rates of the stored templates, dynamic programming is used to determine which samples of a spoken word shall be correlated with which samples of a template along a curve of "warped time" so as to compensate for rate differences.
Besides the field of voice recognition, a dynamic programming algorithm has been used by Applicant to synchronize a receiver to frequencies of low-SNR, narrowband, frequency-hopping satellite signals that were subject to unknown frequency errors by simultaneously testing postulates of hop frequency and frequency error. Using a fast Fourier transform (FFT) processor, a plurality of parallel receivers tuned to different hypothesized frequencies were created simultaneously, and energies from corresponding bins of successive FFTs were accumulated based on knowledge of the hopping pattern. Some aspects of this system are described in U.S. Pat. No. 4,476,566 to Dent.
Prior methods and apparatus did not address reception of wideband signals at low SNR, including the problem of synchronization with a TDMA signal containing a regular succession of known bit patterns that are separated from one another by uncertain time periods. Accordingly, systems and methods are needed for locating and time synchronizing with time slots received with low SNR, even as low as -10 dB.